350 


|       ILLINOIS  STATE  I 

|        LABORATORY  | 

♦    OF  NATURAL  HISTORY  | 

LIBRARY  I 


L  I  E>  R.ARY 

OF  THE 

UN  I  VERSITY 

Of    ILLINOIS 


WfURMr-  H I  STGFef  SURVEY 

550.5 

FI 
v.3,  cop.Ct 


Remote  ST^B4i3 


LIBRARY 

UNIVERSITY  OF  ILLINOIS 

URBANA 


Field  Columbian  Museum. 

Publication  hi. 

Geological  Series.  Vol.  Ill,  No.  3. 


NEW  FORMS  OF  CONCRETIONS 

BY 

Henry  Windsor  Nichols, 
Assistant  Curator,  Department  of  Geology. 


Oliver  Cummings  Farrington,   Ph.  D. 
Curator,  Department  of  Geology. 


.si 


Chicago,  U.  S.  A. 
June  1,  1906. 


LIBRARY 
UNIVERSITY  OF  ILLINOIS 

URSANA 


NEW  FORMS  OF  CONCRETIONS 


By  HENRY  WINDSOR  NICHOLS 


SAND-CALCITE  CONCRETIONS  FROM  SALTON,  CALIFORNIA 

A  series  of  five  sand-calcite  concretions  (Museum  No.  G,  1301) 
presented  to  the  Museum  by  Mr.  Herbert  Brown  of  Yuma,  Arizona, 
appear  worthy  of  description.  Regarding  the  conditions  of  the  occur- 
rence of  these  concretions  little  is  known.  Mr.  Brown  simply  states 
that  they  were  handed  him  by  a  commercial  traveler  as  having 
been  obtained  by  him  at  Salton,  California.  As  there  are  extremely 
large  sand  dunes  in  the  immediate  vicinity  of  Salton,  it  is  probable 
that  the  concretions  were  formed  in  these.  Whether  or  not  the  form 
represented  by  the  specimens  at  hand  is  a  common  or  an  unusual 
type  in  that  locality  is  unknown.  These  concretions  (Plate  XIX) 
are  formed  of  sand  cemented  by  calcite,  and  are,  therefore,  of  the 
type  of  the  well  known  Fontainebleau  and  Saratoga  Springs  concre- 
tions, from  which,  however,  they  differ  in  several  respects.  The  Salton 
concretions  take  the  form  of  an  irregularly  botryoidal  ball  from  which 
projects  a  stout,  tapering  stem  in  such  wise  that  the  object  assumes 
the  shape  and  proportions  of  an  ancient  mace.  The  change  from 
head  to  stem  is  abrupt,  much  as  if  the  stem  were  driven  into  a  hole 
bored  in  the  head,  and  there  is  even  a  slight  annular  depression  in  the 
latter  where  the  stem  enters.  The  botryoidal  appearance  of  the  head 
is  due  to  a  compound  structure  —  each  head  being  built  up  from  a 
number  of  spheroidal  concretions  grown  together.  While  there  is  but 
little  flattening  of  the  concretion  as  a  whole,  the  subordinate  spheroids 
are  much  flattened  and  also  elongated  in  the  line  of  the  principal  axis 
of  the  concretion.  The  specimens  have  a  very  rough  surface  from 
the  presence  in  large  numbers  of  rhombohedral  points  of  arenaceous 
calcite  crystals.  These  points  suggest  that  these  concretions,  like 
those  from  Devil  Hill,  Wyoming,  described  by  Barbour,*  are  aggre- 
gates of  moderately  large  crystals.  Lines  of  stratification  (PlateXIX) 
intersect  the  specimens  in  such  a  direction  as  to  ind  cate  that  the 
principal  axes  lie  conformably  with  the  strata  in  which  they  form. 

*  Bull.  Geol.  Soc.  Amer,  Vol.  XII,  p.  i6s- 

25 


26  Field  Columbian  Museum — Geology,  Vol.  III. 

The  slight  flattening  of  the  complete  individual  as  well  as  the  greater 
flattening  of  the  subordinate  spheroids  of  the  head  is  in  the  plane  of 
bedding  of  the  surrounding  sand. 

The  specimens  in  the  possession  of  the  Museum  weigh  from  45 
to  952  grams.  The  diameter  of  the  ball  lies  between  30  and  70 
millimeters,  and  that  of  the  thickest  part  of  the  stem  between  20  and 
30  millimeters.  The  head  of  the  concretion,  therefore,  varies  much 
more  in  size  than  the  stem.  The  stems,  however,  are  very  variable 
in  length;  the  shortest  is  55  and  the  longest  210  millimeters.  Two  of 
the  specimens  are  compound,  consisting  respectively  of  two  and  three 
individuals  grown  together. 

The  specific  gravity  of  the  concretions  is  2.69,  and  they  are 
therefore  a  little  denser  than  the  average  concretion  of  this  character. 
Concretions  of  sand  and  calcite  from  Saratoga  Springs  in  the  Museum 
collections  have  a  density  of  2.62;  those  from  Fontainebleau  of  2.42. 
The  sand-calcite  concretions  and  crystals  from  Devil  Hill,  Wyoming, 
which  have  been  studied  by  Barbour,*  have  a  specific  gravity,  as 
determined  by  the  present  writer,  of  2.64.  According  to  Dana,  the 
Fontainebleau  crystals  vary  in  specific  gravity  from  2.53  to  2.84.! 
The  great  variation  in  these  figures  is,  however,  not  to  be  taken  as 
indicating  corresponding  variations  in  the  true  density  of  the  objects. 
They  rather  indicate  differences  in  the  methods  employed  by  various 
experimenters  and  differences  in  the  shape  and  size  of  the  pores  of 
different  specimens.  It  is  evident  that  the  true  specific  gravity  of  a 
mixture  of  calcite  and  quartz  cannot  be  less  than  2.65,  the  specific 
gravity  of  the  lighter  constituent.  The  very  great  influence  of  the 
character  of  the  pores  and  of  the  shape  and  size  of  permeable  objects 
of  the  character  of  those  under  consideration  are  discussed  in  this  paper, 
page  50.  For  the  reasons  there  given,  the  specific  gravities  of  the 
Salton,  Fontainebleau,  and  Saratoga  Springs  specimens,  determined  at 
the  Museum,  are  probably  low,  but  it  is  believed  only  slightly  so. 

The  carbonate  cement  of  the  Salton  concretions  is  soluble  rapidly 
and  with  brisk  effervescence  in  cold  dilute  hydrochloric  acid,  and  is 
therefore  essentially  calcite.  The  dissolved  cement,  however,  yie'ds 
noticeable  quantities  of  iron  to  chemical  tests. 

The  sand  of  the  Salton  concretions,  when  cleansed  by  cold  dilute 
hydrochloric  acid,  is  of  a  light  gray  color,  subangular,  and  very  fine. 
It  all  passes  a  60  mesh  sieve,  17%  is  retained  upon  an  80  mesh, 
48%  additional  upon  100  mesh,  and  35%  passes  through  a  sieve  of  100 

♦Bull.  Geol.  Soc.  Am.,  Vol.  XII.  p.  165. 
t  Dana :  System  of  Mineralogy,  p.  266. 


June,   1906.     New  Forms  of  Concretions — Nichols.  27 

meshes  to  the  inch.  It  appears  that  the  closeness  with  which  the 
sand  packs  itself  has  some  bearing  upon  the  nature  of  the  concretion. 
A  sand  of  similar  physical  constitution  was  prepared  from  a  mixture 
of  glass  sands  by  the  use  of  sieves.  This  sand  was  packed  into  a 
glass  cylinder  and  compacted  by  long  tapping  of  the  outside  of  the 
cylinder  by  a  stout  wooden  rod.  This  sand,  so  compacted,  enclosed 
between  its  grains  40%  of  voids  which  were  calculated  by  the  usual 
formula.* 

Such  a  sand  undisturbed  in  its  natural  bed  may  be  assumed  to 
compact  itself  in  time  somewhat  more  than  it  may  be  compacted  by  a 
few  minutes'  tapping  in  the  laboratory.  Such  undisturbed  sand  beds, 
according  to  King  and  others, f  contain  35%  to  40%  of  pore  space. 
Therefore  if  a  sand-calcite  concretion  is  composed  of  calcite  filling 
voids  previously  existent  between  grains  of  sand,  it  will  have  by 
volume  a  composition  of  calcite  35-40%,  silica  65-60%.  The  com- 
position by  weight  will  be  approximately  the  same,  as  the  specific 
gravities  of  the  minerals  differ  but  little.  Such  a  composition  has  in 
fact  been  proved  by  the  only  two  determinations  of  this  character 
known  to  the  author  for  similar  concretions.  These  were  carried  out 
upon  material  from  the  two  widely  separated  localities  Devil  Hill, 
Wyoming,^  and  Fontainebleau,  France. § 

A  determination  of  the  percentage  of  sand  and  calcite  in  the 
Salton  concretions  was  made  upon  material  broken  from  the  stem. 
The  fragments  were  treated  with  cold  dilute  hydrochloric  acid  and  the 
insoluble  sand  weighed.  The  concretion  was  found  to  contain:  sand, 
20.17%;  calcite,  70.83%.  This  corresponds  to  a  composition  by 
volume  of  about:  calcite,  70%;  sand,  30%.  The  above  facts  may  be 
tabulated  as  follows: 

Composition    by  Volume   of   Sand-Calcite    Concretions   from 
Three   Localities: 

Sand,  %  Calcite,  % 

Theory 65-60     35  -  40 

Devil  Hill 64 36 

Fontainebleau 50-63    50  -  3 7 

Salton     30 70 

From  this  table  it  appears  that  in  the  Fontainebleau  and  Devil 
Hill  concretions  the  calcite  is  little,  if  any,  in  excess  of  that  required 
to  fill  the  voids  between  the  sand  grains.  The  Salton  concretions,  on 
the  other  hand,  have  but  half  the  sand  and  twice  the  calcite  required  for 

♦King:  Physics  of  Agriculture,  p.  115. 

tlbid.:  p.  126;  Warington:    Physical  Properties  of  Soil,  p.  66. 
%  Barbour :  Bull.  Geol.  Soc.  Am.,  Vol.  XII,  p.  165. 
§  Dana:    System  of  Mineralogy,  6th  ed.,  p.  266. 


28  Field  Columbian   Museum — Geology,  Vol.  III. 

such  a  constitution.  There  are  four  hypotheses  which  may  account 
for  this  excess  of  calcite:  i.  The  concretion  may  have  formed  in  a 
partially  opened  crevice;     2.  Part  of  the  calcite  may  be  fragmental; 

3.  Part  of  the  sand  may  be  impregnated  with  or  replaced  by  calcite; 

4.  The  calcite  when  crystallizing  may  have  exerted  pressure  upon  the 
sand  grains  and  moved  them  apart. 

The  first  hypothesis,  a  partially  opened  fissure,  is  practically 
negatived  by  many  conditions  and  may  be  dismissed  at  once.  Be- 
tween the  other  three,  microscopic  study  might  discriminate.  A 
slide  was  therefore  prepared  for  this  purpose,  from  a  cross  section  of 
the  stem  of  a  concretion.  The  sand  grains  in  this  slide  proved  to  be  of 
the  usual  character  of  those  sands  which  are  derived  from  acid  crystal- 
line rocks.  The  great  majority  of  the  grains  were  quartz.  Partially 
kaolinized  feldspars  were  present  in  some  quantity,  also  scattered 
fragments  of  biotite,  muscovite,  dark  amphiboles,  and  a  few  grains  of 
minerals  not  readily  recognized.  Such  minerals  as  garnet,  ilmenite, 
magnetite,  etc.,  were  completely  absent.  The  grains  varied  from 
angular  to  well-rounded,  but  the  greater  portion  were  of  a  sub-angular 
character.  With  the  exception  of  the  slight  kaolinization  of  the  eld- 
spars  the  minerals  of  the  sand  grains  were  wholly  unaltered.  The  cal- 
cite proved  to  be  wholly  in  the  cement,  and  the  cement  contained  no 
other  mineral  than  calcite.  No  alteration  of  the  calcite  was  observed, 
nor  any  calcite  of  fragmental  origin,  nor  did  any  of  it  replace  sand. 
The  calcite  was  found  to  occupy  more  than  half  the  area  of  the  slide, 
the  grains  of  sand  seldom  touched,  but  were  separated  by  bands  of 
calcite  cement,  which  varied  greatly  in  width.  These  calcite  bands 
were  frequently  much  wider  than  the  diameter  of  the  enclosed  grains. 
It  appears,  therefore,  that  the  calcite  in  crystallizing  has  exerted 
sufficient  pressure  to  push  apart  the  sand  fragments,  although  no 
anomalous  optical  features  were  noted  indicating  strain  in  the  cement. 
The  cement  was  in  the  form  of  calcite  crystals  of  cross  sections  com- 
parable in  magnitude  with  those  of  the  sand  grains.  While  many  of 
them  lay  in  parallel  positions;  sufficient  data  could  not  be  secured 
from  a  study  of  the  slide  to  determine  whether  or  not  the  calcite  is  in 
the  form  of  radiating  crystals  or  of  other  regular  or  irregular  aggre- 
gates. 

The  concretions  in  the  Museum  collections  which  possess  a 
character  most  resembling  the  Salton  forms  in  shape  and  appearance 
are  from  the  two  well-known  localities:  the  Paris  Basin,  and  Saratoga 
Springs,  New  York.  The  specimen  from  the  Paris  Basin  which 
appears  to  possess  the  most  in  common  with  the  Salton  concretions 


June,  1906.       New  Forms  of  Concretions — Nichols.  29 

is  a  chain  of  four  sand-calcite  balls  from  Clermont.  (Plate  XX,  Fig.  1.) 
This  consists  of  four  spheres  between  140  and  160  millimeters  in 
diameter  united  into  a  slightly  curved  chain  49  centimeters  long. 
The  spheres  where  they  join  interpenetrate  for  perhaps  one-eighth  to 
one-twentieth  of  their  respective  diameters.  Each  ball  s  nearly 
spherical  with  no  marked  flattening  and  is  simple.  The  only  compli- 
cation of  form  is  an  abrupt  change  in  diameter  of  the  spheres  giving 
each  the  external  form  of  a  laminated  body  from  which  the  external 
shell  has  been  half  broken  away.  This  is,  however,  a  consequence  of 
differing  rate  of  growth  for  different  sides  of  the  sphere  and  is  in  no 
wise  dependent  upon  internal  structure.  These  deposits,  which  are 
associated  with  mineral  springs,  are  doubtless  more  or  less  tufaceous 
in  character. 

The  sand-calcite  concretions  of  Saratoga  Springs,  New  York, 
tend  to  form  sheets  by  the  coalescence  of  many  ind.viduals  and  thus 
much  of  the  material  is  better  described  as  sandy  calcareous  tufa 
than  as  concretionary.  The  two  specimens  shown  in  Plate  XXI 
illustrate  this  phase.  These  are  respectively  15  x  40  and  17  x  20 
centimeters  in  area  and  both  are  from  3  to  6  centimeters  thick.  Both 
specimens  are  fragments  evidently  broken  from  consideraMy  larger 
sheets.  The  individual  concretions  from  these  sheets  are  forms 
modified  from  the  sphere  by  agencies  which  have  p  oduced  a  flatten- 
ing and  elongation,  so  that  the  simplest  form  of  common  occurrence 
is  a  somewhat  flattened  ovoid  or  pear  (Plate  XXI,  Fig.  1)  with  the 
same  appearance  of  lamination  which  occurs  upon  the  Paris  Basin 
specimens.  The  larger  number  of  those  concretions  which  unite  to 
form  a  sheet  of  tufa  at  Saratoga  Springs  are  much  more  elongated 
than  these  pear  shapes.  Many  of  these  more  elongated  forms  so 
coalesce  as  to  lose  their  identity  and  present  merely  a  solid,  wavy 
surface.  When  the  individuality  is  not  so  completely  lost,  there 
arise,  first,  shapes  resembling  a  long-necked  gourd,  then,  as  the 
elongation  becomes  greater,  the  flattening  becomes  greater  also,  the 
form  becomes  wavy  in  both  the  horizontal  and  vertical  planes  and 
deep,  strong,  longitudinal  and  occasionally  transverse  striations 
appear.  Thus  the  elongated  individuals  forming  these  sheet-like 
bodies  of  concretion  tend  to  become  flat  and  more  or   less   curved. 

Besides  the  tufaceous  sheets,  separate  individual  concretions 
are  common  among  the  Saratoga  Springs  material.  These  show 
little  or  no  flattening  and  sometimes  but  little  departure  of  any  kind 
from  a  spherical  form.  They  are  frequently  heavily  striated  in  a 
meridional  direction   by  deep  grooves  which  come  together  at  two 


3° 


Field  Columbian  Museum — Geology,  Vol.  III. 


poles.  When  compounded,  they  assume  grotesque  and  imitative 
forms.  The  nearest  approach  to  the  Salton  forms  is  a  double  con- 
cretion from  Saratoga  Springs  (Plate  XX,  Fig.  2).  This  consists  of 
two  cones  with  hemispherical  bases.  They  are  similar  in  form  but 
differ  in  size.  The  apex  of  the  smaller  is  united  with  the  base  of 
the  larger.  The  length  of  the  specimen  is  38  centimeters  and  its 
greatest  diameter  8  centimeters.  This  may  be  considered  as  two 
independent  concretions  which  have  grown  together,  and  the  larger 
cone  alone  may  be  compared  with  the  California  specimens.  This 
larger  cone  is  as  smooth  on  the  surface  as  the  sand  which  enters  into 
its  composition  will  permit.  It  is  slightly  curved.  There  is  the  usual 
fold-like  longitudinal  swelling  where  it  has  grown  faster  in  one 
direction  than  another.  The  cone  tapers  gradually  with  no  abrupt 
change  of  curve  from  the  widest  portion  to  the  apex.  The  relation 
between  the  specimens  from  Salton  and  those  from  Saratoga  Springs 
and  the  Paris  Basin  are  best  brought  out  in  tabular  form: 

Comparison  between  Sand-Calcite  Concretions  from  Three 
Localities: 


.     Salton. 

Saratoga  and  the 
Paris  Basin. 

Surface: 

Roughened  with  rhombo- 

hedral  points.     Never 

striated. 

Smooth.      Often  striated. 

Spherical 
forms: 

Compound.      Oblate. 

Simple. 

Prolate  or  ovoid. 

Pseudo-concentric. 

Elongated 
forms. 

Circular  section. 
Straight. 

Flattened  section. 
Curved  or  wavy. 

Junction  of    • 
spherical  to 
elongated 
form. 

Abrupt. 

Always  gradual. 

Lack  of  data  prevents  discussion  of  the  nature  or  origin  of  these 
concretions  from  Salton,  California.  There  is,  however,  one  sugges- 
tion which  is  called  forth  by  the  shape  of  these  objects  when  they 
are  compared  with  some  hitherto  unrecorded  forms  of  concretions 
of  an'  entirely  different  character.  The  stem  of  any  one  of  these 
California  specimens  is  very  like  a  stalactite  depending  from  the  head. 

In  certain  sand  dunes,  notably  in  the  "Hoosier  Slide ' '  of  Michigan 
City,  Indiana,  flat  sheet-like  bodies  of  limonite  concretion  form  in 
certain  strata  of  the  sand.  When  these  are  dug  out,  numerous  small 
stalactites  of  limonite  are  found  depending  from  their  lower  surfaces. 


June,  1906.     New  Forms  of  Concretions — Nichols.  31 

These  stalactites  are,  however,  too  friable  to  be  preserved.  These 
limonite  concretions  form  by  deposition  from  a  sheet  of  ferriferous 
water  which  flows  during  wet  weather  along  a  more  permeable  layer  of 
dune  sand  or  upon  the  surface  of  a  comparatively  close-packed  and 
impervious  stratum.  It  is  evident  that  this  comparatively  imper- 
meable layer  is  able  to  form  in  wet  weather  some  fashion  of  floor  for 
the  stream  of  iron-bearing  waters.  This  floor  is,  however,  but 
imperfect  and  very  leaky,  so  that  the  limonite  stalactites  have  ample 
opportunity  to  form  where  the  water  drips  through.  It  is  very 
possible  that  in  the  case  of  these  sand-calcite  concretions  some 
similar  structure  of  the  dunes  near  Salton  has  permitted  a  similar 
stalactite  to  form  at  the  base  of  such  concretions  as  were  favorably 
placed. 

SAND-BARITE  CRYSTALS  FROM  OKLAHOMA 


These  specimens  (Museum  No.  G.  1285,  Plate  XXII)  were 
collected  by  Prof.  Charles  N.  Gould  of  the  University  of  Oklahoma 
and  presented  by  him  to  this  Museum.  They  are  found,  according  to 
Prof.  Gould,  along  the  outcrop  of  a  belt  of  red  sandstone  in  Eastern 
Oklahoma.  This  belt  is  about  ten  miles  wide  and  extends  for  a 
distance  of  fifty  or  seventy-five  miles '  through  several  counties, 
particularly  Cleveland,  Oklahoma  and  Lincoln  counties.  Prof. 
Gould  referred  to  the  specimens  in  conversation  as  "sand  crystals." 
Dr.  Otto  Kuntze  in  a  similar  way  calls  them  "barite  pseudomorphs. " 
In  the  catalogue  of  a  Western  mineral  dealer  they  are  listed  as  identical 
with  certain  " silico-barite  concretions"  collected  in  Kansas.  An 
Eastern  dealer  calls  them  "gypsum  pseudomorphs."  It  may  be 
inferred  from  these  differing  appellations  that  there  is  more  than  a 
little  uncertainty  regarding  the  nature  of  these  objects. 

Twelve  specimens  which  came  into  the  posssesion  of  the  Museum 
at  the  close  of  the  St.  Louis  Exposition  vary  from  2%  to  7  centimeters 
in  diameter  and  from  io>£  to  364  grams  in  weight.  They  assume  the 
form  of  rosettes  which  are  composed  of  aggregates  of  tabular  crystals 
resembling  lamellar-nodular  aggregates  of  gypsum,  barite  and  other 
minerals.  The  faces  of  the  plates  are,  however,  somewhat  rounded 
on  the  edges  as  if  eroded  and  hence  not  sufficiently  definite  in  form  to 
permit  of  exact  measurements  or  determination.  According  to 
Prof.  Gould  they  vary  in  size  from  that  of  a  pea  to  a  diameter  of  five 
inches.  They  are  found  both  enclosed  in  the  sandstone  and  weath- 
ered out. 


32  Field  Columbian  Museum — Geology,  Vol.  III. 

A  series  of  32  specimens  received  later  confirms  the  characters  of 
the  earlier  lot.  They  include  a  number  of  globular  specimens  which, 
however,  have  the  same  structure  as  the  rosette  forms,  from  which 
they  differ  in  the  number  and  dimensions  of  the  component  plates. 
That  is,  the  globular  forms  are  merely  thick  rosettes.  One  specimen 
consists  of  a  group  of  many  nearly  globular  forms  enclosed  in  the 
weathered  matrix  which  assumes  the  form  of  a  red  sand.  This  sand 
appears  to  be  the  residue  left  from  solution  of  the  limonite  cement  of 
a  ferruginous  sandstone. 

The  rosette  appears  upon  both  sides  of  an  approximately  octa- 
gonal plate  which  may  be  designated  the  basal  plate  of  the  aggregate. 
This  is  penetrated  obliquely  by  a  variable  number  of  similar  plates 
which  appear  to  intersect  at  the  centre  of  the  aggregate  and  project 
on  both  surfaces.  These  plates  make  angles  of  approximately  300 
with  the  bases.  While  these  plates  appear  as  if  passing  through 
the  basal  plate  and  any  important  one  appearing  on  one  side 
may  be  readily  discovered  on  the  other,  yet  the  two  rosettes  are 
never  exactly  alike.  One  is  always  more  complex  than  the  other  and 
formed  of  smaller  plates.  These  plates  generally,  but  not  always,  lie 
in  a  confusedly  whorled  position.  They  are  not  simple  but  frequently 
consist  of  two  plates  inclined  to  each  other  at  angles  of  approximately 
300  and  intersecting  some  in  the  vertical  and  some  in  the  horizontal 
plane.  By  repetition  of  this  compounding  of  plates,  always  at 
angles  of  approximately  300  so  far  as  the  roughness  of  the 
material  will  allow  determination,  the  apparently  irregular  orientation 
of  the  leaves  of  the  rosettes  may  be  accounted  for.  By  a  greater 
degree  of  this  compounding  also  is  the  greater  complexity  of  one  face 
over  the  other  produced.  The  specimens,  examined  detail  by  detail, 
are  decidedly  unsymmetrical,  yet  when  the  broader  features  only  are 
considered,  symmetry  of  a  high  order  is  present.  The  rosettes  on 
either  side  of  the  basal  plate  while  not  identical  in  detail  are  so  in 
mass,  and  proportioned  so  that  the  aggregates  are  symmetrical  with 
respect  to  the  plane  of  the  basal  plate,  as  well  as  to  a  central  axis  at 
right  angles  to  this  plane.  There  is  also  a  tendency  in  some  of  the 
specimens  towards  an  axis  of  hexagonal  symmetry  in  the  plane  of  the 
basal  plate.  The  secondary  plates  appear  to  so  twist  as  to  all  intersect 
along  this  axis. 

The  position  of  those  portions  of  the  plates  which  lie  buried  in 
the  body  of  the  specimen  may  be  followed  by  the  cleavages  upon  the 
fractured  surfaces.  From  an  examination  of  these  cleavages  it 
becomes  evident  that  the  plates  do  not  really  intersect  or  interpene- 


June,  1906.     New  Forms  of  Concretions — Nichols.  33 

trate.  While  the  projecting  and  visible  portions  are  plane,  that 
portion  of  each  plate  which  is  buried  in  the  mass  of  the  aggregate  is 
invariably  curved  and  frequently  very  strongly  so.  Hence  a  plate 
that  appears  from  the  general  form  to  pass  through  the  basal  plate 
frequently  curves  sharply  into  almost  a  U  shape,  with  both  sides 
projecting  upon  the  same  side  of  the  specimen  while  another  similar 
U-shaped  plate  lies  symmetrically  in  the  opposite  rosette.  Other 
plates  upon  approaching  plates  that  they  appear  to  penetrate,  termi- 
nate there  in  a  wedge,  and  a  similar  form  symmetrically  placed  gives 
the  appearance  of  a  penetration  that  does  not  exist.  In  some  in- 
stances the  aggregations  are  double.  One  specimen  consists  of  two 
rosettes  in  parallel  position  which  have  simply  touched  each  other 
and  adhered.  Another  consists  of  two  individuals  at  right  angles 
which  have  grown  together  giving  the  effect  of  a  more  or  less  spiral, 
elongated  form. 

The  exterior  of  the  specimens  is  of  dark  reddish-brown  color, 
while  the  interior  is  of  a  pale  pink  closely  resembling  the  color  of  some 
pink  orthoclases.  When  broken  a  good  cleavage  developes  in  the 
form  of  a  minute  step  structure  of  very  brilliant  facets  in  parallel 
position  with  pronounced  pearly  lustre.  When  the  fracture  is  ex- 
amined under  the  magnifying  glass  the  cleavage  is  obscured  by  a 
granular  structure  which  is  exactly  that  of  a  broken  face  of  sand 
stone.  The  specimen  is  obviously  composed  of  grains  of  sand  ce- 
mented by  a  mineral  which  possesses  an  eminent  cleavage  in  at  least 
two  directions.  The  average  specific  gravity  of  the  nodules  is  3.348. 
The  individuals  do  not  vary  greatly  in  density  from  this  mean.  The 
color  is  discharged  upon  intense  ignition  but  returns  upon  cooling. 
The  color  after  ignition   however,  is  fainter  than  before. 

A  slide  was  prepared  and  studied  under  the  microscope.  This 
appeared  as  an  aggregate  of  angular  quartz  fragments  of  several  sizes 
enclosed  by  a  cementing  mineral  which  completely  filled  all  voids  or 
interspaces  between  the  quartz.  The  quartz  grains  were  surrounded 
by  a  thin  red  coating  which  resolved  under  high  power  into  groups  of 
brownish-red  isotropic  spherules  and  ellipsoids  upon  the  surface  and 
in  the  fractures  of  the  quartz  grains.  The  granular  fragmental 
material  was  almost  wholly  quartz.  One  small,  isotropic  fragment 
of  yellow  color,  high  refraction  and  no  visible  cleavage,  presumably 
garnet  and  one  good  sized  fragment  of  clouded  orthoclase  appeared. 

The  cement  was  an  anisotropic  mineral  of  two  cleavages,  one 
better  defined  than  the  other,  which  lie  at  an  angle  of  900.  There 
was  a  third  cleavage  parallel  or  nearly  so,  with  the  plane  of  the  slide 


34  Field  Columbian   Museum — Geology,  Vol.  III. 

which  did  not  appear  as  cracks  upon  the  surface  of  the  section.  The 
extinction  was  parallel  to  the  principal  cleavage,  which  lies  in  the 
plane  of  the  axis  of  least  elasticity.  The  index  of  refraction  of  this 
mineral  was  greater  than  that  of  the  quartz.  The  cement  throughout 
the  entire  slide  was  part  of  one  crystal  with  the  growth  of  which  the 
sand  grains  present  had  not  interfered.  This  was  indicated  by  the 
cleavage,  which  was  everywhere  parallel  with  itself,  and  by  the 
interference  color  which  was  the  same  hroughout  the  slide.  The 
high  specific  gravity  of  the  specimen  and  the  presence  of  much  barium 
sulphate,  taken  with  the  features  shown  in  the  slide  indicate  that  this 
cement  is  barite.  In  this  slide  it  was  evidently  cut  parallel  to  m  and 
showed  the  usual  cleavage  parallel  to  c  and  one  set  parallel  to  m. 
An  analysis  of  the  specimens  made  in  the  Museum- laboratories 
by  the  author  gives  the  following  result : 

Si02 36 .  99 

BaO 35.76 

S03 19. 20 

Fe20, 0.82 

Al2Oa 5-36 

CaO 0.51 

MgO 0.03 

H20* ' ,       0.27 

Organicf 0.32 

99.26 
This  corresponds  with  a  mineral  composition  (disregarding  the 
silica  required  for  the  aluminous  minerals)  of: 

Barite '   54.42 

Quartz    36.99 

Miscellaneous 8.59 

100 .00 
From  the  analysis  it  would  appear  that  some  aluminous  mineral 
is  present  bu ;  the  slides  fail  to  disclose  such  in  quantities  required  to 
satisfy  the  analysis.  Inasmuch  as  barite  frequently  contains 
similar  elements  as'  mpurities  even  when  well  crystallized,  it  appears 
best  to  provisionally  include  the  minor  elements  in  the  barite  for  an 
approximate  determination  of  mineral  composition.  The  mineral 
composition  thus  becomes: 

Barite 63 

Quartz 37 

100 
This   corresponds   to   a   specific   gravity   of   3.77    against   3.380 

*  From  air-dried  specimen,  by  Penfield's  method, 
t  Loss  on  ignition  less  water. 


Juxe,  1906.     New  Forms  of  Concretions — Nichols.  35 

actually  found  for  the  individual  from  which  the  material  for  the 
analysis  was  taken.  This  discrepancy  would  be  too  great  were  it  not 
for  the  fact,  elsewhere  discussed  in  these  papers,  that  the  specific 
gravity  determined  for  these  mineral  aggregates  is  commonly  too 
low  owing  to  air  trapped  in  pores,  cracks,  etc.,  which  cannot  be 
wholly  removed  by  boiling  or  by  the  air  pump.  If,  however,  we 
assume  that  all  the  bases  except  the  barite  are  in  the  form  of  silicates 
which  have  a  density  equal  to  quartz,  the  calculated  density  3.62  is 
but  slightly  lower  than  that  before  obtained. 

By  the  method  described  on  page  27,  the  space  occupied  by 
the  quartz  and  barite  may  be  calculated.  The  calculation  so  made 
shows  that  the  quartz  occupies  50%  of  the  volume  of  the  concretion 
and  the  bar;te  50%.  As  sand  naturally  packed  generally  ncludes 
about  40%  of  voids  between  the  grains,  it  appears  as  if  the  barite  had 
crystallized  between  the  grains  of  sand  and  very  slightly  pushed  them 
apart  by  pressure  when  growing.  Indeed  there  are  in  the  slide 
examined,  here  and  there  a  few  evidences  of  slight  pressure  upon  the 
cement  in  the  shape  of  a  rise  in  the  order  of  interference  color  com- 
bined with  a  wavy  extinction.  These  spots  however  are  very  few 
and  very  small. 

These  specimens  are,  therefore,  not  concretions  in  the  narrow 
sense  of  the  term,  but  crystal  aggregates  of  barite  with  sand  present 
as  a  mechanically  held  impurity.  They  bear  the  same  relation  to  the 
known  occurrences  of  sandstone  with  barite  cement  that  the  sand- 
calcite  crystals  of  Fontainebleau  and  Devil  Hill  do  to  the  sandstones 
with  calcareous  cement. 

L1MONITE-SAND  CONCRETIONS,  SPRING  LAKE,  MICHIGAN 


These  concretions  (Museum  No.  G.  1223,  Plate  XXIII)  were 
collected  at  Spring  Lake,  Michigan,  by  the  author.  They  occur  on 
the  tops  of  dunes  where  the  sand  has  been  overgrown  with  grasses  and 
shrubs.  In  places  the  vegetation  has  disappeared  and  the  sand  has 
again  begun  to  move.  Thus  there  are  formed  shallow  pits  where  the 
surface  has  been  removed  to  depths  of  from  an  inch  or  two  to  five  or 
six  feet  below  the  sod.  These  concretions  lie  on  the  surface  of  these 
pits  in  the  loose  sand.  From  the  shallowness  of  some  of  these  pits, 
it  is  evident  that  many  of  the  concretions  must  be  formed  within  a 
few  inches  of  the  original  sodded  surface  of  the  dune.  Inasmuch  as 
in  the  deeper  pits  the  supply  of  concretions  is  not  perceptibly  greater 
than  in  the  shallowest  of  all,  it  appears  that  few,  if  any,  of  the  concre- 


36  Field  Columbian  Museum — Geology,  Vol.  III. 

tions  originate  at  any  considerable  depth  below  the  surface.  The 
concretions  are  irregular,  lumpy  forms  without  approach  to  any 
regularity  or  symmetry  beyond  the  fact  that  the  majority  of  them 
are  more  or  less  flattened  and  many  have  one  flat  side.  They  are 
occasionally  penetrated  by  minute  cylindrical  holes  up  to  2  mm. 
in  diameter  such  as  would  be  the  case  if  they  had  been  penetrated  by 
rootlets.  They  are  of  reddish-brown  limonite  color  rarely  approaching 
a  hematite-red  in  places.  They  are  but  slightly  consolidated  and 
may  be  readily  reduced  to  their  constituent  sand  grains  by  pressure  of 
the  fingers.  They  do  not  commonly  exceed  5  centimeters  in 
any  dimension.  In  composition  they  are  dune  sand  cemented  by  a 
small  proportion  of  limonite  which  does  not  fill  the  voids  between  the 
grains.  The  limonite  is  merely  a  coating  on  the  sand  grains.  When- 
ever the  grains  touch  their  coatings'  coalesce,  thus  cementing  the 
sands  into  a  concretion.  There  is  no  evidence  of  any  nucleus  in  any 
of  the  specimens  examined  nor  is  there  any  determinable  concentric 
structure. 

There  is  no  mystery  about  the  origin  of  these  forms  beyond  the 
determination  of  which  of  three  or  four  common  agents  has  been  the 
predominant  precipitant  of  the  cement.  The  sand  of  the  dunes  in 
which  they  were  found  is,  like  nearly  all  dune  and  beach  sand,  of  a 
yellowish -brown  color.  This  color  is  due  to  a  thin  coating  of  limonite. 
Where  the,  dunes  have  not  been  fixed  by  vegetation,  this  color  is  not 
noticeably  lighter  at  the  surface  than  it  is  in  depth.  Where  a  dune  is 
fixed  by  vegetation  a  light  sod  often  forms  over  the  surface.  Under 
this  sod  the  sand  is  much  lighter  in  color  for  a  depth  of  a  few  inches 
than  it  is  at  greater  depth.  Hence  it  is  to  be  inferred  that  the 
organic  compounds  derived  from  the  vegetation  have,  as  is  cus- 
tomary, dissolved  the  iron  oxides  from  that  sand  which  lies  immediately 
under  the  sod.  From  organic  compounds  containing  iron  dissolved  in 
the  so-called  humus  acids,  the  metal  is  rapidly  precipitated  by  any 
one  of  several  agents,  the  more  common  of  which  are  spontaneous 
changes  in  the  organic  solvent,  bacterial  action,  oxidation  and  hydroly- 
sis. The  hydrated  ferric  oxide  precipitated  is  deposited  by  preference 
as  a '  film  upon  the  surface  of  the  sand  grains  and  by  spontaneous 
dehydration  forms  the  limonite  cement. 

As  he  precipitation  has  fol'owed  so  immediately  on  solution  as 
to  produce  concretions  within  a  few  inches  of  the  surface  it  is  pro- 
bable that  the  precipitating  agent  is  either  air  in  the  pores  between 
the  sand  grains,  iron-secreting  bacteria,  or  more  probably  a  hydro- 
lization  of  iron  compounds  of  weak  organic  acids  consequent  upon 


June,  1906.     New  Forms  of  Concretions — Nichols.  37 

large  dilution  of  the  solvent  when  removed  from  the  immediate 
vicinity  of  the  decaying  root  or  leaf  which  is  the  1  ource  of  its  supply. 
Such  small  limonite-sand  concretions  forming  near  the  surface 
of  semi-fixed  dunes  are,  therefore,  due  to  an  action  of  vegetation  upon 
the  limonite  coatings  of  the  sand  grains  of  the  dune,  an  origin  not 
unlike  that  of  the  bog  and  pond  limonites. 

LIMONITE  GEODES,  MUSCOGEE,  INDIAN  TERRITORY 


A  series  of  limonite  geodes  (Museum  No.  G.  1308)  of  unusual 
character  was  presented  to  the  Museum  by  General  G.  Murray  Guion. 
According  to  General  Guion  the  geodes  are  found  in  clay  in  the  bottom 
of  a  "draw"  or  ravine  at  Muscogee,  Indian  Territory.  These  spec- 
imens are  composed  essentially  of  limonite  with  turgite  and  consist  of 
a  crust,  a  core  and  a  central  cavity.  They  are  of  the  irregular  discoid 
form  with  smooth  exterior  which  characterizes  a  common  type  of 
siderite  nodule.  They  are  of  moderate  size.  A  typical  specimen 
(Plate  XXIV,  Fig.  4)  weighs  270  grams,  has  a  diameter  which  varies 
from  10  to  12  centimeters  and  a  thickness  of  4  centimeters.  When 
the  specimen  lies  flat  its  horizontal  projection  is  a  decidedly  irregular 
oval.  All  vertical  projections  and  sections  are  ovals,  slightly  irregular 
but  symmetrical  with  respect  to  the  major  diameter.  Some  specimens 
possess  th  cker  and  some  thinner  forms  than  this.  The  surface  is 
smooth  except  for  such  roughness  as  is  due  to  scaling  of  the  lamellar 
crust.  The  color  is  light  gray  with  dark  brown  stains.  Some 
specimens  are  coated  with  a  firmly  adherent  yellow  ochreous  clay  in 
which  they  appear  to  have  been  imbedded,  while  many  specimens 
are  perfectly  free  from  this  coating.  The  specimen  shown  in  the 
illustration  is  enclosed  in  a  light-colored  laminated  crust.  Inside  the 
crust  and  sharply  separated  therefrom,  is  the  main  portion  of  the 
geode,  a  hard,  red  and  yellow,  concentrically  banded,  agate-like  mass 
of  limonite  and  turgite.  The  center  is  occupied  by  a  small  cavity 
which  varies  in  shape  and  size  in  different  specimens,  and  suggests  in 
outlines  the  central  cavity  often  found  in  agates. 

The  shell  is  from  3  to  7  millimeters  in  thickness.  Its  external 
color  is  gray  to  brown;  fractured  surfaces  are  ight  gray  with  dark 
brown  and  limonite  yellow  areas.  The  outer  portion  of  the  crust  is 
almost  universally  light  gray,  while  the  inner  parts  contain  more 
of  the  darker  areas. 

The  crust  is  strongly  laminated,  especially  in  the  outer  portions. 
The  individual  laminae,  which  are  somewhat  under  a  millimeter  in 


38  Field  Columbian  Museum — Geology,  Vol.   III. 

thickness,  are  very  brittle  and  break  readily  in  some  instances  into 
little,  straight-sided  rhombs  which  are  not  uniform  in  shape  or  size. 
The  hardness  of  this  crust  is  about  that  of  calcite.  In  appearance 
the  material  of  this  crust  resembles  a  siderite  partially  altered  to 
limonite.  A  chemical  test,  however,  proves  it  to  be  limonite  mixed 
with  clay  and  a  very  little  calcite. 

Inside  this  shell  is  the  core,  which  comprises  the  principal  mass  of 
the  specimen.  This  core  readily  separates  from  the  shell  when  the 
geode  is  broken.  It  consists  of  hard  red  turgite,  banded  concen- 
trically with  limonite.  (Plate  XXIV,  Fig.  3.)  The  red  portion 
forms  by  far  the  larger  part  of  the  core.  The  hardness  of  the  core 
like  that  of  the  crust  is  about  that  of  calcite.  This  core  is  of  a  smooth, 
earthy  texture.  It  rubs  off  sufficiently  to  soil  paper  readily.  The 
agate-like  banding  is  disposed  somewhat  symmetrically  with  reference 
to  the  centre  and  the  outside.  A  section  of  the  core  presents  an 
annular  form.  The  centre  of  this  ring  is  occupied  by  a  broad  red 
band,  outside  and  inside  of  which  are  thin,  alternating  bands  of 
yellow  and  red,  while  the  broad  central  red  band  is  itself  made  up  of 
a  multitude  of  minute,  almost  invisible  bands  of  two  shades  of  red. 

The  central  cavity  is  small  in  proportion  to  the  size  of  the  geode. 
One  specimen  which  has  been  sawn  through  the  centre  presents  a 
section  of  an  average  diameter  of  6  centimeters.  In  this  specimen 
the  section  of  the  cavity  occupies  a  space  of  15  by  5  millimeters. 
•The  section  of  the  opening  has  the  form  of  an  irregular  pentagon 
with  sharp  angles  suggesting  a  crystal  outline  which  is  common 
among  agates.  The  cavity  in  this  instance  has  a  dark  brown,  slightly 
iridescent  coating  of  botryoidal  limonite  with  two  small  areas  of 
colorless,  transparent  opal  also  botryoidal  A  thinner  specimen 
of  about  5  by  25  millimeters  section  when  sawn  through  the  centre 
reveals  the  central  cavity  reduced  to  a  mere  slit  of  2  by  10 
millimeters.  This  cavity  is  in  the  red  turgite  and  has  no  limonite 
coating.  It  has,  however,  a  partial  coating  of  an  opaque  white 
powder,  the  nature  of  which  has  not  been  determined. 

Composed  of  quartz,  these  specimens  would  be  typical  agates. 
Therefore  it  is  most  probable  that  they  were  formed  in  the  same  way 
as  agates  by  the  deposition  of  oxides  of  iron  instead  of  silica.  As  in 
the  case  of  agates  slight  changes  in  the  conditions  of  deposition  cause 
changes  in  the  color  and  porosity  of  silica  deposited,  so  in  this  instance 
slight  changes  in  the  surroundings  or  in  the  mother  liquor  have 
caused  alternate  depositions  of  more  and  less  hydra  ted  oxides  of  iron. 
Further  discussion  of  the  origin  and  nature  of  these  objects  would 


June,  1906.     New  Forms  of  Concretions — Nichols.  39 

appear  unprofitable  until  their  occurrence  has  been  investigated  in 
the  field. 


LIMONITE  GEODES  FROM  THE  OHIO  RIVER 


A  series  of  four  hollow  limonite  objects  (Museum  No.  G.  1307) 
of  rhombohedral  form  which  were  presented  to  the  Museum  by  Dr. 
W.  S.  Gilmore  prove  to  be  limonite  geodes.  (Plate  XXIV,  Figs  1 
and  2.)  They  are  described  as  occurring  in  large  numbers  in  clay 
upon  the  banks  of  the  Ohio  River  about  30  miles  from  Owensboro,  Ky. 

They  are  small,  weighing  from  28  to  64  grams.  They  are  all  of 
approximately  the  same  thickness,  25  millimeters,  the  same  width, 
25  millimeters  and  vary  in  length  from  26  to  60  millimeters.  With 
the  exception  of  one  imperfect  specimen  they  are  bounded  by  plane 
faces  and  are  in  form  typical  joint  rhombohedrons  formed  between 
bedding  planes  and  three  systems  of  parallel  and  intersecting  joints 
perpendicular  to  the  bedding.  Two  systems  of  the  joints  are  practi- 
cally perpendicular  to  each  other.  The  third  system  intersects  the 
others  at  angles  varying  from  400  to  6o°.  In  all  the  specimens  two 
parallel  surfaces  which  differ  in  color  from,  and  are  more  earthy  in 
texture  and  rougher  than  the  others,  are  identified  as  bedding  planes. 

The  surface  of  the  geodes  is  yellow  on  the  bedding  planes 
and  dull  red  to  brown  on  the  joint  faces.  Fractured  surfaces  are 
dull  brown  and  smooth,  with  a  yellow  streak  at  the  inside  edge.  The 
specimens  are  hollow,  with  thicker  walls  along  the  bedding  planes 
than  along  the  joint  surfaces. 

In  one  specimen  (Plate  XXIV,  Fig.  2)  the  walls  of  the  geode  in 
contact  with  the  bedding  planes  have  a  thickness  of  5  to  7  millimeters, 
while  the  walls  in  contact  with  joint  planes  have  a  thickness  of  only  1  to 
2  millimeters.  This  specimen  happens  to  be  double,  the  half-specimen 
or  individual  to  which  the  above  measurements  refer  having  a  breadth 
and  thickness  respectively  of  24  and  16  millimeters  The  interior 
hollows  of  the  unbroken  geodes  are  filled  with  a  tough,. yellow,  och- 
reous  clay,  reticulated  on  the  surface  with  drying  cracks. 

It  is  very  ev  dent  from  the  form  and  structure  of  these  objects 
that  they  are  formed  at  the  intersection  of  joints  and  bedding  planes. 
They  do  not  represent  actual  open  spaces,  but  rather  are  blocks  of 
clay  enclosed  by  these  fractures  and  modified  by  the  introduction  of 
limonite  from  the  exterior  by  ferruginous  waters.  These  waters  do 
not  appear  to  have  deposited  their  iron  in  the  joint  openings  them- 


40  Field  Columbian  Museum — Geology,  Vol.  III. 

selves  to  any  considerable  extent,  as  in  this  case  there  would  be- 
instead  of  individual  geodes,  a  cellular  honeycomb  structure  of 
limonite  enclosing  clay  in  its  meshes. 

The  limonite  has  been  deposited  principally,  perhaps  wholly, 
where  the  ferruginous  waters  have  soaked  into  the  clay  as  coatings 
upon  the  individual  clay  particles.  Not  filling  the  joint  fractures,  the 
limonite  coatings  of  adjacent  specimens  do  not  commonly  adhere. 
When  they  do  adhere,  compound  or  twin  geodes  are  formed.  The 
source  of  the  iron  cannot  be  determined,  as  practically  nothing  is 
known  of  the  mode  of  occurrence  of  these  objects.  Except  for  the 
outer  form,  these  objects  simulate  closely  those  concretions  that  are 
assumed  to  originate  in  the  decomposition  of  a  pyr'.te  nodule  and  the 
deposition  of  the  resultant  oxide  of  iron  around  it.  It  is  a  question 
if  many  of  the  hollow  iron  concretions  may  not  be  geodes  of  this 
nature,  although  it  is  certain  that  not  all  are.  If  the  deposition  of 
iron  oxide  continued  long  enough,  such  a  deposit  would  become  one 
of  argillaceous  limonite. 

NODULES    FROM.  THE  CHALLENGER  AND  ARGUS  BANKS  IN 
THE  ATLANTIC  OCEAN 


While  engaged  in  collecting  fish  for  this  Museum,  Dr.  Tarleton 
H.  Bean,  on  the  12th  of  October,  1905,  dredged  from  the  Challenger 
Bank  sixty- four  calcareous  nodules.  The  following  day  he  dredged 
from  the  Argus  Bank  twenty-eight  similar  nodules.  These  specimens, 
now  a  part  of  the  Museum  collections  (Museum  Nos.  G.  1323-30),  are, 
sufficiently  problematic  in  character  to  be  worthy  of  some  study, 
especially  as,  if  of  a  certain  character,  they  would  have  an  important 
bearing  upon  geological  and  geographical  problems  of  great  interest. 

The  Challenger  Bank,  whence  the  larger  number  of  specimens 
were  secured,  is  a  shoal  of  from  five  to  ten  miles  diameter,  rising 
abruptly  from  the  depths  of  the  sea  to  within  twenty-four  to  thirty 
fathoms  from  the  surface.  The  Bank  lies  thirteen  miles  southwest 
of  Gibbs  Lighthouse,  Bermuda,  and  is  separated  from  the  Bermuda 
Bank  by  a  space  of  three  and  one  half  miles  of  deep  sea,  where 
soundings  exceeding  1,000  fathoms  have  been  taken.  The  Argus 
Bank  is  a  shoal  of  similar  dimensions  and  depth  Of  water  about 
twelve  miles  southwest  of  the  Challenger  Bank,  from  which  it  is 
separated  by  a  trough  of  five  hundred  fathoms  depth.  There  is  no 
shallow  water  connection  between  these  two  banks,  nor  with  any 
other  shoals  or  land. 


June,  1906.     New   Forms  of  Concretions — Nichols.  41 

It  was  the  opinion  of  "James  D.  Dana*  that  these  two  banks  were, 
in  comparatively  recent  historical  times,  islands,  which  were  even 
mapped  as  "The  False  Bermudas."  Early  accounts  of  these  banks 
described  them  as  "rocky  ledges,  "f  The  ship  Challenger  visited  the 
bank  of  that  name  upon  the  23d  of  April,  1873.  Upon  its  map  of  the 
region+  the  character  of  the  bank  is  given  as  coral.  Sir  C.  Wyville 
Thompson,§  who  was  with  the  Challenger  expedition,  says:  "The 
bank,  which  seems  to  be  about  five  miles  across,  consists  mainly  of 
large  rounded  pebbles  of  the  substance  of  the  Bermuda  serpuline  reef. 
There  is  an  abundant  growth  all  over  the  pebbles  of  the  pretty  little 
branching  corals,  Madracis  asperula  and  M.  hellana. "  He  mentions 
also  that  starfish  and  other  animals  were  brought  up  in  the  dredge. 
Mr.  Bean,  dredging  in  28  fathoms,  found  that  the  bottom  was 
covered  with  the  nodules  under  consideration,  which  are  doubtless 
identical  with  Sir  Wyville  Thompson's  pebbles.  The  nodules  were, 
however,  imbedded  in  calcareous  ooze,  and  although  covered  by 
living  forms,  the  branching  skeletons,  which  may  well  correspond 
with  Madracis,  appear  from  inspection  of  the  dried  specimens  to 
have  been  dead  sufficiently  long  to  become  encrusted  with  bryozoa 
and  nullipora. 

If  these  nodules  are  rolled  fragments  of  serpuline  limestone,  both 
the  existence  within  a  few  hundred  years  of  the  False  Bermudas  and 
their  extremely  rapid  subsidence  is  as  good  as  proven.  The  three 
and  one  half  miles  of  deep  sea  which  separate  the  banks  from  the 
nearest  reefs  offer  an  insuperable  obstacle  to  the  transportation  of 
pebbles  in  such  large  numbers.  Such  nodules  of  fragmental  origin 
also  could  not  form  in  situ  under  present  conditions,  for  wave  action 
at  depths  of  twenty-four  to  thirty  fathoms  is  either  very  weak  or 
entirely  lacking.  The  current  of  three  knots  has  not  sufficient 
power  to  round  boulders  of  such  size.  If,  however,  they  are 
accretions,  they  have  little  or  no  apparent  bearing  upon  these  ques- 
tions, and  the  interest  in  them  arises  from  other  sources. 

The  nodules  from  the  Challenger  bank  (Plate  XXV)  in  the 
possession  of  the  Museum  were  dredged,  as  already  stated,  from  a 
depth  of  about  twenty-eight  fathoms.  The  nodules  are  roughly 
spherical,  with  pitted  and  irregular  surfaces.  When  collected,  they 
were  covered  with  living  hydrozoa,  other  animal  forms  and  algae.     The 

♦Corals  and  Coral  Islands,  p.  187. 

tlbid. 

JChallenger  Report:  Narrative  :  Vol.  I,  facing  p.  149. 

SVoyage  of  the  Challenger :    The  Atlantic,  Vol.  I,  p.  333. 


42  Field  Columbian  Museum — Geology,  Vol.  III. 

nodules  are  of  a  light  cream  color,  with  membranous  patches  of 
red  and  brown  dried  organic  matter,  (Meloboesia)  which  continues  to 
produce  the  characteristic  pungent  odor  of  drying  marine  vegetation. 
From  the  weights  and  dimensions  of  56  individuals,  the  writer  has  cal- 
culated the  average  size  and  shape.  It  is  a  rounded  body,  like  a  slightly 
crushed  sphere,  9.9  centimeters  long,  8.7  centimeters  wide,  and  7.6 
centimeters  thick.  •  Its  weight  is  340  grams.  The  variation  of  size 
in  the  nodules  at  hand  is  considerable,  the  maximum  diameters  of 
the  56  individuals  lying  between  6.8  and  14  centimeters,  and  the 
minimum  diameters  between  5.7  and  11.2  centimeters.  The  corres- 
ponding weights  are  118  and  940  grams.  Between  these  limits  the 
sizes  and  weights  of  the  nodules  are  distributed  with  a  fair  degree  of 
uniformity.  While  the  individual  nodules  frequently  depart  far 
from  a  true  spherical  form,  they  appear  on  casual  inspection  to  do 
so  in  all  possible  directions,  and  with  no  tendency  toward  any  other 
definite  shape  than  the  sphere.  When  the  dimensions  of  all  the 
specimens  are  tabulated,  however,  it  appears  at  once  that  the 
majority  of  the  forms  are  such  that  the  three  perpendicular  axes  or 
diameters  are  unequal,  and  the  length  of  the  intermediate  is  an 
arithmetical  mean  between  the  longest  and  the  shortest  diameter. 
The  average  nodule,  as  described  above,  also  has  this  form.  Few  of 
the  specimens  depart  far  from  these  proportions.  Many,  however, 
while  maintaining  their  ratios  between  major,  median,  and  minor  axes 
do  depart  materially  from  the  form  of  the  spheroid  of  the  same  axes. 
One  is  in  the  form  of  a  cone  with  a  large,  shallow  depression  in  its 
base,  well  to  one  side  of  its  axis.  (Plate  XXVI,  Fig.  6.)  Others  have 
slight  flattenings  and  concavities  which  are  suggestive.  Frequently, 
on  the  flatter  sides  of  the  nodule  there  will  be  slight  depressions  a 
little  to  one  side  of  the  centre.  These  forms  dimly  but  persistently 
suggest  half-obliterated  fofms  of  familiar  gastropod  shells.  (Plate 
XXVI.) 

The  specific  gravity  of  a  specimen  weighing  540  grams  was 
found  to  be  2.30.* 

The  surface  of  the  nodule  (Plate  XXV)  is  always  of  an  irregularly 
rough  or  warty  appearance.  It  is  composed  entirely  of  the  skeletons 
of  calcareous  encrusting  organisms  which  are  chiefly 'corals,  bryozoa 
and  algae.  In  places  the  surface  is  covered  with  cylindrical  branch- 
ing forms,  (Madracis  f )  which  may  attain  a  height  of  8  mm.  and  a  dia- 
meter for  individual  cylinders  of  perhaps  2  mm.  These  forms  were 
all  dead  when  the  specimens  were  collected,  and  are  in  all  instances 

*  See  p.  50  for  cause  of  low  results. 


June,  1906.     New  Forms  of  Concretions — Nichols.  43 

thickly  covered,  and  partially  obliterated  by  other  incrustations. 
Other  portions  are  covered  with  somewhat  smaller  club-shaped  branch- 
ing forms  of  bryozoa.  The  entire  surfaces  of  all  the  specimens  are 
covered  with  films  of  encrusting  bryozoa,  and  of  a  nullipore  allied  to 
Meloboesia  of  which  many  were  living  when  the  nodules  were  col- 
lected. The  surfaces  show  also  a  multitude  of  forms  of  other 
calcareous  organisms,  including  curved  worm  tubes,  fan-like  forms, 
etc.,  of  occasional  occurrence.  The  specimens  collected  in  1873 
by  the  Challenger  were  covered  with  living  Madracis,  which 
appear  in  the  present  specimens  to  have  been  replaced  for  the  most 
part  by  nullipora  and  by  bryozoa  of  encrusting  rather  than  branching 
forms.  The  larger  branching  corals,  etc.,  are  confined  to  one-half  of 
the  surface,  the  other  half  being  fairly  smooth,  and  coated  only  with 
the  smoother  encrusting  forms.  This  smooth  half  probably  is  the 
part  embedded  in  the  calcareous  ooze  from  which  the  nodules  were 
dredged.  To  some  specimens  are  attached  completely  encrusted 
shells  of  sizes  up  to  45  millimeters.  '(Plate  XXVI,  Fig.  4.)  The 
nodules  are  penetrated  frequently  by  syphon  tubes  of  a  Pholas-like 
shell.  These  shells  were  all  dead  when  collected  and  filled  with 
calcareous  sand.  Some  of  the  boring  mussels  are  also  represented  by 
long-dead  shells.  There  are  also  numerous  serpula-like  calcareous 
tubes  penetrating  the  nodules  in  every  direction.  The  calcite  of  the 
surface  is  of  a  friable,  chalky,  and  earthy  character,  giving  no 
indications  of  macroscopic  crystallization.  For  purpose  of  study 
several  specimens  were  sawn  through  the  centre  with  a  hack- 
saw. These  sections  (Plate  XXVII)  exhibit  a  chalky,  cellular  lime- 
stone, becoming  more  solid  and  denser  toward  the  centre.  The  cells 
possess  no  regularity  in  form,  size,  or  distribution.  Some  of  the 
openings  are  sections  of  the  syphons  Of  Pholas  or  some  allied  form,  of 
worm  tubes  and  of  pelecypod  shells;  more  are  merely  irregular 
cavities  in  the  limestone.  Towards  the  centre  the  cells  are  smaller, 
with  thicker  walls,  and  toward  the  surface  they  are  larger,  with 
thinner  walls.  Upon  examination  from  a  distance,  the  cells  have  a 
distinctly  concentric  arrangement,  which  disappears  upon  close 
examination,  except  near  the  surface.  Close  to  the  surface,  and  for  a 
distance  inward  of  six  or  eight  millimeters,  the  material  is  in  the  form 
of  concentric,  irregularly  waved  sheets  of  calcite,  which  touch  and 
coalesce  in  spots  enclosing  elongated,  empty  cells  lying  approximately 
parallel  with  the  surface.  Upon  the  outside  of  the  nodules  there  are, 
in  places,  thin  encrusting  bryozoa  and  algae,  which  arch  away  from  the 
nodule  in  a  similar  manner.     This  type  of  cellular  structure  dies  out 


44  Field  Columbian  Museum — Geology,  Vol.  III. 

gradually  toward  the  center  by  a  thickening  of  the  walls  and  a 
shortening  of  the  cells  to  approximately  equidimensional  forms. 

The  calcite  of  the  body  of  the  nodule  is  continuous  with  that  of 
the  encrusting  forms.  There  are  certain  exceptions  to  this,  however. 
A  nodule  (Plate  XXVI,  Fig.  2,  and  Plate  XXVII,  Fig.  2)  of  a  shape 
suggesting  an  enclosed  shell,  and  about  ten  centimeters  in  diameter, 
when  opened  disclosed  the  very  light  cellular  calcite  to  a  thickness 
of  ten  to  fifteen  millimeters,  and  enclosing  an  annular  core  of  denser 
cream-colored  rock  with  a  line  of  demarcation  perfectly  sharp.  This 
hard  material  has  the  shape  of  a  curved  loop  three  millimeters  thick, 
inside  of  which  the  cellular  material  again  occurs.  It  appears  to  be 
a  section  through  the  shell  of  a  large  gastropod.  Other  sections  of 
specimens  display  the  same  character.  Nearly  all  the  nodules  which 
were  opened  contained  shells  of  Pholas,  or  of  some  allied  form.  Some 
specimens  of  the  boring  mussels  were  found,  as  well.  The  Pholas- 
like  shells  had  been  dead  for  some  time,  and  were  filled  with  calcite 
sand,  but  the  syphon  tubes  penetrated  in  every  instance  either  quite 
to  the  surface  or  to  within  one  millimeter  of  it.  Small  gastropods 
completely  enclosed  and  the  calcareous  tubes  of  worms  are  of  fre- 
quent occurrence. 

Such  are  the  nodules  from  the  Challenger  Bank.  The  twenty- 
eight  specimens  collected  October  13,  1905,  from  the  neighboring 
Argus  Bank  are  of  the  same  general  character,  but  differ  in  some 
respects.  They  are  smaller,  and  much  more  irregular  in  outline,  as 
well  as  darker  colored  from  the  presence  of  the  calcareous  algae  in  larger 
numbers.  They  came  from  a  depth  of  from  28  to  30  fathoms.  They 
vary  in  diameter  from  three  to  eight  centimeters,  and  in  weight  from  5 
to  212  grams.  Some  of  the  nodules  have  the  spheroidal  form  of  those 
from  the  Challenger  Bank,  but  many  have  no  regularity  of  shape  what- 
ever. Some  of  them  are  simply  shapeless  intergrowths  of  branching 
coralline  forms,  and  others  appear  to  be  encrustations  upon  flat  shells 
of  various  shapes  and  sizes.  The  variety  in  form  and  size  of  the  speci- 
mens from  the  Argus  Bank  is  well  demonstrated  by  the  specimens 
forming  the  bottom  row  of  Plate  XXV. 

A  chemical  analysis  of  the  substance  of  a  nodule  seeming  desirable, 
the  inner  part  of  an  average  specimen  of  about  eight  centimeters 
diameter  from  the  Challenger  Bank  was  taken  for  the  purpose.  After 
about  one  centimeter  had  been  removed  from  the  outside  by  chipping, 
the  remaining  portion  was  pulverized  to  pass  a  40-mesh  sieve  and 
quartered  down  to  convenient  bulk  for  analysis.  The  analysis  by 
the  author  gave  the  following  result: 


June,  1906.       New  Forms  of  Concretions — Nichols.  45 

Analysis  of  the  Centre  of  a  Nodule  from  the  Challenger 

Bank. 

CaO   49-66 

CO?    42.92 

MgO 2.38 

Na20    0.34 

MnO    0.05 

FeO      0.12 

Al2Oa     0.58 

Si02     o .  n 

SO,      0.55 

P205     0.02 

CI   0.37 

Loss  in  ignition* 2 .  93 

100.03 
Less  O  A  Cl2 o .  08 

99  •  95 
This  corresponds  to: 

Calcium  carbonate    88 .  61 

Magnesium  carbonate 4-98 

Ferrous  carbonate , 0.21 

Manganese  carbonate     o .  08 

Miscellaneous, 6 .07 

99-95 
A  magnesia  determination  was  also  made  upon  about  two  grams 
of  the  extreme  outer  portion  of  the  nodule.  5.12%  of  magnesia  was 
found,  corresponding  with  10.70%  of  carbonate  of  magnesia.  Thus 
it  appears  that  the  exterior  of  the  nodule  is  more  magnesian  than  the 
interior. 

Inasmuch  as  the  nodules  occur  isolated  on  a  small  bank  in  the 
midst  of  the  Atlantic,  away  from  any  possibility  of  impregnation 
or  alteration  by  waters  flowing  from  pre-existing  mineral  veins,  the 
presence  or  absence  of  minute  proportions  of  the  heavy  metals  is  of 
importance  as  it  bears  directly  upon  the  much  disputed  question  of 
the  origin  of  ore  deposits  by  lateral  secretion  or  ascension.  The 
isolation  of  the  material  removes  wholly  the  serious  doubt  present  in 
most  determinations  of  this  character  as  to  whether  any  metals 
found  may  not  have  originated  in  mineral  veins  and  later  impregnated 
the  surrounding  rock.  Consequently  a  search  for  traces  of  copper 
and  lead  in  thirty-seven  grams  of  the  nodule  material  was  carried  out 
with  great  care.     There  was  not  a  trace  of  either  metal  present. 

*LessC02.     Chiefly  organic  matter  and  some  water.    The  organic  matter  makes  itself  very 
evident  upon  igniting  the  specimen,  both  by  its  odor  and  by  blackening. 


46  Field  Columbian  Museum — Geology,  Vol.  III. 

The  magnesian  character  of  so  recently  formed  a  limestone  of 
organic  origin  is  somewhat  unexpected;  even  though  analyses  of 
reef  rock,  coral  limestone,  and  coquina  invariably  show  magnesia  in 
similar  quantity.  The  composition  of  these  nodules  is  essentially 
that  of  the  Bahama  reefs  and  of  other  limestones  of  comparatively 
recent  organic  origin.*  There  are  ancient  crystalline  marbles  (e.g. 
Vermont)  which  are  shown  by  analysis  to  have  a  similar  constitution 
as  regards  magnesia. 

While  the  source  of  the  magnesia  is  undoubtedly  the  magnesian 
salts  in  sea  water,  the  modus  operandi  of  the  transfer  from  the  sea 
salt  to  the  nodule  appears  doubtful.  There  are  three  possible  methods: 
i.  Formation  of  the  nodules  by  direct  chemical  precipitation  of 
the  two  carbonates;  2.  Metasomatic  replacement  of  calcium  by 
magnesium;  3.  Secretion  of  magnesium  carbonate  with  the  lime  by 
organisms.  The  present  tendency  of  geological  belief  is  towards  the 
replacement  hypothesis,  although  there  are  yet  those  who  believe  the 
older  dolomites  are  direct  chemical  precipitates.  The  application 
of  the  theory  of  replacement  of  lime  by  magnesia  to  the  present  case 
meets  serious  objections. 

Experimental  studies  of  the  replacement  of  calcium  by  magnesium 
in  carbonates  indicate  that  under  certain  abnormal  conditions  of 
pressure  and  temperature  such  replacements  readily  occur,  f  Also 
a  co-precipitation  of  carbonates  of  lime  and  magnesia  may  be 
produced  under  conditions  of  concentration  of  the  mother  liquor 
which  cause  it  to  differ  widely  from  sea  water  in  character.  On  the 
other  hand,  experiments  by  Bischof,^  and  others  have  indicated  that 
under  normal  conditions  either  such  replacement  does  not  occur  or 
takes  place  so  slowly  that  an  experiment  of  several  years'  duration 
yields  no  perceptible  result.  So  eminent  an  authority  as  Mendeleef, 
however,  states  that  such  replacement  can  occur  and  will  proceed  until 
a  condition  of  equilibrium  dependent  upon  concentration  and  temper- 
ature is  attained. §  Such  an  origin  of  dolomitic  limestones  necessarily 
postulates  that  they  are  formed  under  two  sets  of  widely  variant  condi- 
tions, under  one  of  which  the  equilibrium  is  reached  at  from  one  to  ten 
per  cent  magnesium  carbonate,  and  under  the  other  the  equilibrium  is 
reached  when  the  magnesium  carbonate  in  the  dolomite  attains  a 
proportion  not  greatly  below  45.65%,  which  corresponds  to  the 
double    salt    MgC03.CaCOs.     Limestones    with    magnesian    content 

*  U.  S.  G.  S.  Bull.  228. 

t  Fouque  et  Levy :  Synthese  des  Mineraux,  p.  204. 

J  Bischof:  Chemical  and  Ph ysical  Geology,  vol.  Ill,  p.  167. 

§Mendel£eff:  Principles  of  Chemistry,  vol.  I,  ch.  14,  footnote  11. 


June,  1906.     New  Forms  of  Coxcretioxs — Nichols.  47 

between  these  limits  are  of  very  unusual  occurrence,  as  are  carbonate 
rocks  with  magnesia  much  in  excess  of  that  in  dolomite.  Experi- 
ments by  various  chemists  and  experimental  geologists  have  amply 
demonstrated  that  a  co-precipitation  of  magnesia  and  lime  car- 
bonates under  normal  conditions  of  concentration,  pressure,  etc., 
is  impossible.  The  depth  of  28  fathoms,  however,  corresponds  to  a 
pressure  of  100  pounds  to  the  square  inch,  more  or  less,  and  under 
this  pressure  and  at  ordinary  temperatures  experiments  have  not  been 
carried  out. 

These  considerations  are  not  intended  to  prove  that  dolomites  and 
magnesian  limestones  are  never  formed  by  metasomatic  processes  or 
by  direct  precipitation.  The  evidences  of  metasomatic  origin  for 
some  dolomitic  limestones  which  have  been  summarized  by  Van  Hise* 
are  convincing.  It  is,  however,  evident  from  the  above  considerations 
that  the  conditions  under  which  the  Bermuda  nodules  grew  are  not 
such  as  favor  either  of  these  processes  of  dolomite  formation.  Inas- 
much as  the  nodules  are  evidently  organic  in  origin,  direct  secretion 
of  magnesia  by  the  organisms  concerned  seems  a  reasonable  hypoth- 
esis, especially  as  such  an  action  would  be  to  the  advantage  of  the 
organism  by  rendering  its  skeleton  more  insoluble.  As  some  brach- 
iopods  and  all  vertebrates  secrete  phosphates,  and  some  sponges, 
diatoms,  etc.,  silica,  there  seems  to  be  no  a  priori  reason  why  corals, 
etc.,  should  not  secrete  carbonate  of  magnesia  together  with  carbonate 
of  lime.  There  appears  to  be  an  impression  which  is  very  wide 
spread  that  all  such  calcareous  skeletons  are  extremely  pure  carbonate 
of  lime,  but  a  cursory  examination  of  available  literature  discloses  no 
grounds  for  such  a  belief.  Dana,  Geikie  and  Prestwichf  quoting 
Dolter  and  Homes'  work  upon  the  dolomites  of  the  Tyrols,  note 
that  some  organically  deposited  limestone  is  slightly  magnesian  at 
the  time  of  formation.  Many  writers  refer  briefly  to  the  work  of 
Forchhammer  discussed  in  the  following  pages,  but  either  minimize 
the  importance  of  his  results  or  fail  to  see  their  significance. 

To  determine  whether  calcareous  organisms  ever  become  magne- 
sian enough  to  account  for  the  character  of  these  nodules  magnesia 
was  determined  by  the  author  in  the  Museum  laboratory  for  twelve 
skeletons  of  calcareous  organisms  of  various  types.  With  the  results 
of  this  work  are  tabulated  twenty-one  determinations  by  other 
analysts.  The  determinations  as  given  in  the  table  are  of  the  speci- 
mens  as   prepared   for  exhibition.     These   naturally   contain   dried 

*  U.  S.  G.  S.  Mon.  XLVII,  p.  802. 

tDana:    Manual  of  Geology,  p.  134;  Geikie:   Textbook  of  Geology,  p.  321;  Prestwich:  Geol. 
Vol.  I,  p.  113. 


48              Field  Columbian  Museum — Geology,  Vol.  III. 

organic  matter  in  considerable  quantities,  so  that  the  ratio  of  magnesia 

to  lime  carbonates  in  the  skeletons  alone  is  higher  than  indicated  by 
the  percentages  obtained  for  the  dried  specimens,  which  latter  percent- 
ages are  the  ones  in  the  accompanying  table. 

Magnesium   Carbonate   Content  of  the   Skeletons  of  Various 
Marine  Calcareous  Organisms. 

Analyses  by  the  author  in  Roman  type ;  those  by  other  analysts 
in  italics. 

No.     Mus.  No.  Mg  C03 

ALCYONOID    CORALS.  % 

'i     377         Eunicea  tourneforti,  Bahamas,  2.78 

2  381         Plexaurella  dichotoma,  Bahamas,  2. 11 

3     I  sis  hippuris*  6.362 

4     Corallium  nobile*  2.132 

5     Corallium  rubrum,  Mediterranean,!  9.32 

6  314        Tubipora  musica,  Singapore,  3.83 

ZOANTHOID    CORALS. 

7  126         Coeloria  daedolea,  Abyssinia,  0.35 

8     Astraea  cellulosa*  0.542 

9 Siderastraea  sp.,  Bermuda,%  0.42 

'     BRYOZOA. 

10     1 04 1       Flustra  foliacea,  California,  1.23 

n Eschar  a  foliacea  *  0.146 

12     1052      Garapholas  sp.,  3.99 

13 Bryozoan?  Bermuda,  5.35 

14  1057       Lithoramnion  racemus,  Bahamas,  0.65 

15 Myriazoon  truncatum,*  °-455 

16 Heteropora  abrotanoides*  0-352 

17     ■      Frondipora  reticulata  *  0.596 

pelecypoda. 

18  2879      Teredo  gigantea,  Indian  Ocean,  0.00 

19     Ostrea  sp.,%  0.3 

20 Modiola  papuana*  °-7°5 

21     Pinna  nigra,  Red  Sea  *  1.000 

GASTROPODA. 

22  G1331    Vermetus  sp.,  Bermuda, |j  0.35 

*  Analysis  by  J.  G.  Forchhammer :  1849.  Bidrag  til  Dolomitens  Dannelshistorie  :  Oversigt  over 
det  Kongelige  Danske  Videnskab.  1849,  pp.  83  -  96. 
t  Polished  material  from  a  necklace. 

X  Analysis  by  L.  G.  Eakins;  Bull.  U.  S.  G.  S.,  No.  228,  p.  308. 
§  Analysis  by  Sharpies  ?  Dana'-  Manual  of  Geology,  p.  72. 
The  incrustation  of  specimen  shown  in  Plate  XXVI,   Fig.  5. 


June,  1906 

New  Forms  of  Concretio 

23 



Tritonium  pompilius  * 

24 

Cerithium  telescopicum,  * 

BRACHIOPODA. 

25 



Lingula  ovalis,jf 

26 



Tercbratula  psittacea* 

VERMES. 

27 

Serpula  sp.  Mediterranean* 

28 



Serpula  triqiieta.     North  Sea* 

29 

Serpula  filograna* 

CRINOIDEA. 

30 

P6877 

Metacrinus  rundus,  Japan,! 

CEPHALOPODA. 

31 



Nautilus  pompilius  * 

32 



Ossa  sepiae  [Sepia  sp.]* 

ALGAE. 

33 



Lithothamnium  nodosum,  § 

-JMICHOLS.  49 

O.486 
O.189 

3-59 
o.452 

7.644 

4-455 
1-349 

11.72 

o  118 
0.401 

5-5 

It  may  also  be  noted  that  Sharpies  when  in  187 1  he  determined 
the  phosphate  in  seven  zoanthoid  corals  observed  that  traces  of 
magnesia  were  present  in  all,  but  made  no  quantitative  determina- 
tions. A.  Damour*!  also  found  small  quantities  of  magnesia  in  many 
millepores. 

From  the  above  results  it  appears  that  the  algae,  crinoidea, 
vermes  and  alcyonaria  secrete  relatively  magnesian  skeletons,  while 
the  zoantharia,  pelecypoda,  gastropoda  and  cephalopoda  secrete 
skeletons  which  are  only  slightly  magnesian. 

These  analyses  thus  explain  why  the  inner  portion  of  the  nodule 
analysed  by  the  author,  (p.  45)  is  less  magnesian  than  the  outer  part. 
This  nodule  like  many  of  the  others  was  formed  in  and  around  a 
large  gastropod.  The  more  highly  magnesian  corals,  serpulae  and 
algae  of  which  the  nodule  is  composed  are  in  the  central  part  diluted 
by  the  less  magnesian  gastropod  material.  It  is  probable  that  the 
magnesium  of  the  outer  part  is  also  somewhat  increased  by  that 
re-solution  of  the   skeletal  material  which  is  always  taking  place. 

If  under  present  conditions  corals,  etc.,  secrete  skeletons  which 
may  contain  over  ten  per  cent  carbonate  of  magnesia,  may  they  not, 
under  palaeozoic  conditions,  when,  as  is  usually  conceded,  the  sea 

tAnalysis  by  T.  Sterry  Hunt :  Logan's  Geology  of  Canada,  1863.  The  ash  analyzed  was  61  per 
cent  of  the  whole  shell  and  gave  2.88  per  cent  Mg  O,  whence  the  equivalent  Mg  CO3  for  the  entire 
shell  has  been  calculated. 

tCirri  and  pinnulate  arms  from  an  alcoholic  specimen.    The  organic  matter  is  22  per  cent. 

^Analysis  by  Gumbel;  Geikie:  Textbook  of  Geology,  p.  482. 
Sharpies:  Am.  J.  Sci.,  Ill  ser.,  vol.  I,  p.  169. 

cDana:  Manual  of  Geology,  p.  72. 


50  Field  Columbian  Museum—Geology,  Vol.  III. 

water  was  very  different  in  composition  and  possibly  far  more  corrosive 
than  at  present,  have  protected  themselves  by  secreting  relatively 
insoluble  dolomite  skeletons? 

From  their  composition  and  structure  it  is  very  evident  that 
these  objects  are  accretions  and  not  rolled  fragments  of  preexisting 
rock.  Therefore  they  have  no  bearing  upon  questions  relating  to 
subsidence.  ;From  the  continuity  between  the  living  covering  of 
the  nodules  and  the  calcite  of  the  interior,  as  well  as  from  the  detection 
under  the  microscope  of  organic  structure  in  this  calcite  it  becomes 
certain  that  the  accretions  are  of  organic  growth.  They  are  not, 
however,  individual  animals,  for  organisms  of  many  kinds  are  inter- 
mingled in  them.  They  owe  their  existence  to  a  sequence  of  events 
substantially  as  follows:  The  surface  of  the  bank  was  covered  with 
a  soft  calcareous  ooze  upon  which  coralline  organisms  could  get 
no  foothold.  Upon  this  ooze  certain  gastropods  and  other  shells 
were  able  to  live.  Also  it  is  possible  that  the  shells  of  dead  animals 
may  be  transported  to  the  bank  by  the  current  of  the  Gulf  Stream. 
The  Challenger  secured  living  starfish  there  and  other  forms  of  li  e. 
Such  gastropod,  echinoid  and  other  shells  provided  the  firm  anchor- 
age denied  by  the  ooze  for  encrusting  calcareous  organisms  of  many 
kinds.  These,  growing  generation  over  generation,  have  built  up  the 
nodules.  If  the  growth  of  the  nodules  is  more  rapid  than  the 
deposition  of  the  ooze,  then  they  will  eventually  coalesce  and  form 
a  surface  from  which  a  coral  reef  may  grow  upwards  toward  the 
surface. 

THE  SPECIFIC  GRAVITY  OF  CLAYSTONES 


When  it  was  attempted  to  compare  the  specific  gravities  of  the 
concretions  herein  described  with  the  densities  of  other  concretions, 
it  was  found  that  apparently  such  densities  had  never  been  deter- 
mined. Therefore  after  the  specific  gravities  of  the  specimens 
strictly  comparable  with  those  under  consideration  had  been  secured, 
the  work  was  continued  by  the  determination  and  comparison  of 
the  densities  of  fifty-four  claystones  from  eight  localities. 

The  specific  gravities  were  obtained  in  the  usual  manner  by 
weighing  in  water  after  immersion  to  complete  saturation.  Clay- 
stones  are  permeable  to  water  and  absorb  it  in  large  quantities,  but, 
after  the  first  few  minutes,  very  slowly.  A  constant  weight  in  water 
is  seldom  attained  with  less  than  twelve  to  twenty-four  hours  immer- 
sion. Frequently  the  weight  is  appreciably  constant  only  after 
treatment  for  several  days. 


June,  1906.      New  Forms  of  Coxcretioxs — Nichols.  51 

Claystones  cannot  be  boiled  to  hasten  saturation  as  they  dis- 
integrate to  a  serious  extent.  For  specimens  of  this  character  the 
use  of  the  air  pump  is  of  but  little  value.  This  very  slow  permeability 
of  partially  saturated  claystones  is  a  necessary  consequence  of  the 
peculiar  mesh-like  structure  already  described  by  Emerson.*  The 
rate  of  absorption  becomes  less  as  the  outer  parts  become  saturated 
until  it  is  so  small  that  increase  in  weight  of  the  specimen 
under  treatment  is  masked  or  imperceptible  for  periods  as  great 
as  24  hours.  The  last  air  of  the  interior  is  trapped  and  can  be 
removed  only  by  solution  in  the  water.  This  solution  is  greatly 
impeded  by  the  slight  mobijity  of  water  confined  in  the  capillary 
spaces  so  that  the  dissolved  air  can  be  removed  by  only  slow  diffusion 
unaided  by  convection  currents  in  the  water.  The  density  obtained 
for  claystones  is  therefore  less  than  the  true  density  by  a  quantity 
which  is  greater  the  thicker  the  specimen.  It  is  undesirable,  however, 
in  order  to  avoid  this  presumably  small  and  regular  error,  to  introduce 
the  error  due  to  solution  of  cement  and  consequent  disintegration  of 
the  surface  which  would  arise  from  too  prolonged  immersion  of  the 
specimen.  This  latter  error  which  is  found  to  be  very  large  and  also 
very  irregular  has  to  be  guarded  against  most  carefully.  This  disinte- 
gration from  the  surface  of  clay  stones  in  water  is  so  great  with 
specimens  from  some  regions  that  all  attempts  to  ascertain  their 
density  proved  futile.  Where  an  abundance  of  material  may  be 
sacrificed  in  the  work,  pycnometer  methods  may  possibly  yield 
results  free  from  these  errors  but  the  experience  of  the  author  has  been 
that  little  dependence  can  be  placed  upon  pycnometer  determinations 
made  upon  such  small  quantities  of  material  as  could  be  sacrificed 
for  this  purpose.  Hence  no  such  determinations  were  made.  The 
specific  gravities  of  the  claystones  examined  are  tabulated  on 
page 

When  the  forms  of  the  specimens  were  compared  with  their 
densities  an  apparent  relationship  between  the  density  and  relative 
thickness  appeared.  To  properly  compare  these  features  a  numerical 
value  for  the  rotundity  or  flatness  of  the  'specimen  is  absolutely 
necessary.  As  a  suitable  expression  for  the  variation  of  form  in  this 
respect  the  term  modulus  of  rotundity  is  proposed.  The  diameter  of 
that  circle  which  has  an  area  equal  to  the  horizontal  projection  of 
the  concretion  is  calculated  or  measured.  This  divided  by  the 
extreme  thickness  gives  the  modulus  of  rotundity,  a  number  which 
is  greater  for  the  thinner  forms  and  which  becomes  unity  for  the 

*U.  S.Geol.  Survey,  Monograph   XXIX,  p.  717. 


52  Field  Columbian  Museum — Geology,  Vol.  III. 

sphere.  This  number  is  of  a  very  convenient  magnitude,  varying 
from  i  to  16.4  for  the  forms  in  the  collections.  This  modulus  is  the 
reciprocal  of  the  coefficient  of  rotundity. 

Table  of  Specific  Gravity  and  Modulus  of  Rotundity  of  Clay- 
stones. 

Locality.  Mus.  No. 

G 
Riga,  Vermont.  -        41-1 

39-i 
66-2 

39-2 

4i-3 

72-3 

"  72-1 

69 
72-2 

39-3 
37-3 
38-i 
3  7-4 
69-4 
40-1 
40-2 
69-2 
38-2 
40-2 

69-3 
37-2 

37-i 
Connecticut  River.  805-6 

805-8 

76-2 

■    "  752-2 

805-7 

70-2 

"  .      .  70-1 

55-2 

75-i 
805-2 

805-4 
805-5 


No. 

S.G. 

M.R 

1 

2.77 

11. 6 

2 

2.76 

8.9 

3 

2.76 

8.2 

4 

2-75 

9.8 

5 

2-75 

11. 1 

6 

2.74 

11. 1 

7 

2.74 

10.6 

8 

2.74 

8.8 

9 

2-73 

6-3 

10 

2.71 

5-1 

11 

2.68 

3-6 

12 

2.68 

2.6 

13 

2.67 

4-i 

14 

2.67 

2.9 

15 

2.66 

2.0 

16 

2.66 

i-7 

17 

2.66 

i-7 

18 

2.65 

3-3 

19 

2.65 

i-7 

20 

2.64 

i-9 

21 

2.63 

3-5 

22 

2.63 

3-o 

23 

2.77 

7-3 

24 

2-73 

8.1 

25 

2-73 

4.6 

26 

2.71 

5-6 

27 

2.71 

1.8 

28 

2.70 

5-9 

29 

2.70 

3-o 

30 

2.69 

4-5 

3i 

2.69 

3-9 

32 

2.69 

2.0 

33 

2.69 

i-5 

34 

2.69 

!-5 

June,  1906 

] 

35 

2.68 

3-4 

36 

2.68 

3 

3 

37 

2.68 

3 

2 

38 

2.68 

2 

9 

39 

2.68 

2 

7 

40 

2.67 

5 

3 

4i 

2.67 

3 

4 

42 

2.67 

3 

4 

43 

2.70 

2 

0 

44 

2.76 

3 

0 

45 

2-73 

1 

6 

46 

2-93 

16 

4 

47 

2.71 

8 

1 

48 

2.67 

2 

2 

49 

2.66 

3 

9 

50 

2.78 

7 

8 

5i 

2.77 

2 

4 

52 

2.76 

2 

3 

53 

2.68 

5 

4 

54 

2.68 

3 

4 

55 

2.63 

1 

4 

New  Forms  of  Concretions — Nichols. 
Connecticut  River 


53 


Hartford,  Connecticut. 
Deerfield,  Massachusetts. 

South  Hadley,  Massachusetts. 
Charleston,  New  Hampshire. 


Cumberland,  Maine. 


Broad  Cove,  Maine. 


805-1 

55-1 

76-3 

805-3 
75-2 
76-1 

Vo-3 
805-9 

75i 

753-2 

753-J 

73 
755-1 
755-3 
755-2 
757-1 
757-2 

757-3 
756-2 

756-i 

756-3 


Of  all  the  specimens  examined  those  from  Riga,  Vermont,  are 
available  in  the  largest  numbers  and  vary  most  in  thickness.  Their 
forms  are  extremely  simple  varying  from  nearly  spherical  to  thin,  wa- 
fer-like disks  with  but  few  irregular  shapes.  They  are  therefore  favor- 
able specimens  for  study.  Of  the  twenty-two  from  this  region 
examined,  the  twelve  with  specific  gravity  below  2.70  have 
a  modulus  below  5.  The  ten  specimens  with  specific  gravity 
above  2.70  have  a  modulus  above  5.  Thus  the  modulus  of 
rotundity  seems  to  increase  in  a  general  way  with  the  density. 
It  is  probable  that  the  increase  in  density  with  increased 
thinness  is  only  apparent  and  is  really  due  to  those  defects  inher- 
ent in  the  "methods  of  determination  which  have  already  been 
stated. 

Clay  stones,  as  impure  concretions,  are  subject  to  many  purely 
fortuitous  variations  in  composition.  It  is  of  importance  to  note 
that  almost  any  such  variations  from  normal  composition  will  give 
a  specimen  of  greater  specific  gravity  than  the  typical  claystone. 
The  glacial  clays  in  which  claystones  commonly  occur  are  rock  flours 
of  varied  composition.  As  a  general  rule  they  consist  essentially  of 
floured  quartz,  kaolin  and  kaolinized  feldspars  and  calcite.     Such 


54  Field  Columbian  Museum — Geology,  Vol.  III. 

clays  have  a  true  specific  gravity  between  2.62  and  2.65.  This  clay 
persists  unchanged  throughout  the  substance  of  all  claystones  formed 
in  it.  Any  pebble  or  other  foreign  substance  in  the  clay  is  enclosed 
by  and  made  a  part  of  any  clay  stone  that  forms  in  the  proper  position. 
With  the  exception  of  quartz  any  pebble  likely  to  be  encountered  in 
concretion-bearing  beds  is  considerably  heavier  than  the  surrounding 
clay.  Bits  of  shell,  frequently  encountered  in  claystones  from  some 
localities,  render  the  concretion  in  which  they  occur  heavier  than 
normal.  Rock  flour  clays  may,  and  frequently  do,  contain  pulverized 
minerals  of  many  species,  practically  all  of  which  are  heavier  than 
the  normal  quartz  and  kaolin.  Spots  and  seams  stained  with  iron 
oxides,  segregations  of  magnetic  iron  sand,  pulverized  hornblende, 
etc.,  are  not  at  all  uncommon.  The  cement  of  a  claystone  is,  so  far  as 
known,  essentially  calcium  carbonate.  Usually  it  is  somewhat 
magnesian  and  occasionally  ferriferous.  In  either  case  the  specific 
gravity  of  the  concretion  is  increased.  Fortuitous  variations  in 
composition  and  structure  therefore  commonly  increase  the  specific 
gravity.  It  is  astonishing  that  in  bodies  apparently  subject  io 
purely  fortuitous  changes  so  many  and  so  great,  this  change  of 
density  with  form  should  not  be  entirely  masked.  That  it  is  not  so 
masked,  suggests  that  there  are  only  narrow  limits  of  structure  and 
quality  of  clay  and  cement  within  which  the  formation  of  these 
concretions  is  possible. 


LIBRARY 

imiVERSIIY  OP  ILLINOIS 

URBANA 


o 

4) 

"  ft 

C  M 

tf.E 

,u  c 

fe  a 

-co 

+j    . 

C   «8 

C  ctf 

u  «- 


Q       .     . 

2         -l     <N 


o  o 

life 


LIBRARY 

UNIVERSITY  OF  ILLINOIS 

URBANA 


UNiVOtolir  01"  ILLINOIS 
UR8ANA 


LIBRARY 
UNIVERSITY  OF  ILLINOIS 

URBANA 


LIBRARY 
UNIVERSITY  OP  ILLINOIS 

URBANA 


FIELD  COLUMBIAN   MUSEUM. 


GEOLOGY,  VOL.  Ill,   PLATE  XXIV. 


LlMONMTE    GEODES,    X     « 

Fig.   i.  Two  adjacent  geodes,  Kentucky. 

Fig.  2.  Section  of  twin  geode.  Kentucky. 

Fig.  3.  Section  of  geode,  Muskogee.  Indian  Territory. 

Fig.  4.  Limonite  geode,  Muskogee,  Indian  Territory. 


LIBRARY 

UNIVERSITY  OF  ILLINOIS 
URBANA 


«.    *  «- 


LIBRARY 

UNIVERSITY  OF  ILLINOIS 

URBANA 


LIBRARY 
UNIVERSITY  OF  ILLINOIS 
URBANA      • 


R 


RM(R'*\ 


w 


